2025 H2 Chem Prelim P3 Worked Solutions CJC

9729/03/CJC JC2 Preliminary Examination 2025 [Turn over CANDIDATE NAME CLASS 2T INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 15 September 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 32 printed pages. For Examiner’s Use Section A Q1 /21 Q2 /20 Q3 /19 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examination Higher 2

2 9729/03/CJC JC2 Preliminary Examination 2025 Section A Answer all the questions in this section. 1 (a) Phenylalanine is an essential -amino acid with the formula C 9H11NO2. It cannot be synthesised by the human body and need to be obtained through diet. Phenylalanine can react with alanine, CH3CH(NH2)CO2H, to form a mixture of dipeptides. (i) Name the type of reaction when phenylalanine reacts with alanine. [1] (ii) State the number of possible different constitutional isomers that can be formed in this reaction. [1] (iii) Draw the structures of the constitutional isomers with the molecular formula C12H16N2O3 that are formed in this reaction. The peptide bond formed in each constitutional isomer should be shown displayed. [2] …………………………………………………….…………………………………………… … …………………………………………………….…………………………………………… … (a) (iii) Phe-Ala: (a)(i) condensation (a)(ii) 2 constitutional isomers peptide bond

3 9729/03/CJC JC2 Preliminary Examination 2025 [Turn over Ala-Phe: …………………………………………………….……………………………………………. .. (b) Table 1.1 shows the pKa values of the different functional groups present in phenylalanine and alanine. Both amino acids exist mainly as zwitterions at pH 7.0. (i) State what is meant by the term zwitterion. [1] Table 1.1 (ii) Draw the predominant species of phenylalanine at pH 1.0. [1] (iii) Suggest a pH at which the predominant species of alanine has a net negative charge. [1] …………………………………………………….…………………………………………… … …………………………………………………….…………………………………………… … (b)(ii) Phenylalanine at pH 1.0 …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. (c) The position of substitution in the electrophilic substitution of mono-substituted arenes depends on the nature of the group already attached to the ring. This selectivity can be explained based on the stability of the intermediate formed in the first step. Fig. 1.1 shows three isomers P, Q, R, with the same molecular formula, C9H11NO2, as phenylalanine that can be formed from an appropriate starting alkylbenzene. Use this information to predict which isomer in Fig. 1.1 will be formed the least and which isomer will be formed the most. Explain your reasoning. [3] amino acid pKa of -carboxyl group pKa of -amino group phenylalanine 1.83 9.13 alanine 2.34 9.87 peptide bond (b)(i) Zwitterion is a dipolar ion and has no net charge. (b)(iii) pH 10.0

4 9729/03/CJC JC2 Preliminary Examination 2025 …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. (c) Formed the least: isomer Q. Formed the most: isomer R In carbocation of isomer Q, the alkyl group which is activating/electron-donating is unable to stabilise the carbocation significantly as the positively charged carbon is not directly bonded to alkyl group, whereas isomers P and R will be formed as major products because the activating alkyl group is able to stabilize the carbocation as the positively charged carbon is directly bonded to the alkyl group. However, isomer R is likely to be formed the most as it does not have steric hindrance between the NO2 and alkyl group, like isomer P does.

5 9729/03/CJC JC2 Preliminary Examination 2025 [Turn over (d) Diazonium salts are commonly used to produce synthetics dyes with intense colours that do not occur naturally. The cation of a diazonium salt can be made by reacting an arylamine, RNH2, with nitrous acid, HNO2. The five stages of the reaction are described in Table 1.2. Table 1.2 (i) Name the type of reaction in stage 1. [1] (ii) Complete the mechanism in stage 1 by adding a lone pair and curly arrows in Table 1.2. [2] (iii) Complete the mechanisms in stage 3 and stage 5 by adding lone pairs and relevant curly arrows. [2] stage description of stage equation 1 2 deprotonation 3 protonation and electron pair movement 4 deprotonation and protonation 5 Electron pair movement and heterolytic cleavage of N–O bond

6 9729/03/CJC JC2 Preliminary Examination 2025 …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. (e) Describe the variation in the acid -base behaviour of the Period 3 oxides by reference to the reactions of MgO, Al2O3 and SiO 2 separately with sulfuric acid, H 2SO4, and with potassium hydroxide, KOH. Write equations for any reactions described. [6] …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. (e) Across Period 3, the nature of the oxides changes from basic to amphoteric to acidic. MgO has giant ionic lattice structure and is basic oxide. It reacts readily with acids to give salts and water. MgO + H2SO4 → MgSO4 + H2O Al2O3 is an ionic oxide with partial covalent character. A l3+, with its high charge densit